Hoag's Object (PGC 54559)

Hoag's Object (PGC 54559) is an unusual ring galaxy in the constellation Serpens, named after the American astronomer
Arthur Hoag who discovered it in 1950. Observations have revealed an intricate
structure in the bright ring, which has a near-circular projected shape
and a diameter of about 100,000 light-years. According to one school of
thought, Hoag's Object is a disk galaxy and the ring was formed from the end of a central bar that has since dissolved.
Countering this is the view that the inner core is an E0 elliptical
galaxy, not a disk, and that the ring resulted from an accretion event
2 to 3 billion years ago. It has also been proposed that Hoag's Object be
considered the prototype of a class called Hoag-type galaxies,
which are neither obviously barred nor obviously inclined disks, and which
have outer rings including a significant fraction of the total luminosity.